1. Field of the Invention:
The present invention relates to continuously cast slabs for production of grain-oriented electrical steel sheets or strips having excellent magnetic properties, particularly grain-oriented electrical steel sheets in which the component grain of body-centered cubic lattice has a grain orientation of (110)[001] by Miller indices.
A high magnetic flux density grain-oriented electrical steel sheet has generally an excellent magnetic flux density value, usually expressed by the B.sub.10 value, and a method for producing the same is disclosed in U.S. Pat. No. 3,636,579. According to the disclosure of the U.S. Patent, it is suggested that a continuously cast slab may be used as a starting material in the proposed method. However, the U.S. Patent discloses or teaches nothing about the continuous casting method.
In recent years, the industrilization of continuous casting techniques has been progressing rapidly, and the continuous casting techniques themselves have been making remarkable progress. Continuous casting processes have technical advantages such as the possibility of producing slabs of uniform chemical composition, hence uniform properties, along the slab length, in addition to commercial advantages such as improved production yield due to simplification of the production steps, and increased productivity by saving labour.
The utilization of continuous casting processes has been made progressively also in the field of the production of grain-oriented electrical steel sheets and resulted in various advantages, but on the other hand such problems have been often encountered that the secondary recrystallization is not complete in the final product, thus resulting in inferior magnetic properties.
In order to overcome the above defects in producing grain-oriented electrical steel sheets by continuous-casting, various technical solutions have been proposed. For instance, U.S. Pat. No. 3,764,406 discloses a method for producing grain-oriented electrical steel sheets by two-step hot rolling of a continuous cast slab, and Japanese Patent Publication Sho 50- 37009, as well as the corresponding U.S. Pat. No. 3,841,924, proposes a method for producing a high magnetic flux density grain-oriented electrical steel sheet similarly by two-step hot rolling of a continuously cast slab.
It should be noted, however, that both of the prior art relates to production of hot bands by two-step hot rolling, not fully utilizing the technical advantages inherent with the continuous casting process.
Final products produced from continuous cast steel slabs are generally susceptible to the imcomple secondary recrystallization portion, usually called "streaks", as pointed out in U.S. Pat. No. 3,764,406, and "Metallurgical Transaction" Vol. 6-A, May 1975, page 1041, and the cause of this "streaks" is attributed to the fact that the grains grow excessively in the slab due to the slab heating prior to the hot rolling, resulting in large elongated grains in the hot bands.
According to the disclosures of U.S. Pat. No. 3,764,406, Japanese Patent Publication Sho 50-37009, as well as U.S. Pat. No. 3,841,924, it is possible to prevent occurrence of the incomplete secondary recrystallization portion in the final product when the excessive grain growth during the slab reheating prior to the second hot rolling is prevented by preheating of the slab and a preliminary hot rolling (prerolling), so as to maintain an average grain size not larger than 7 mm in diameter according to U.S. Pat. No. 3,764,406, and to maintain at least 80% in the size of an average grain size less than 25 mm in diameter according to Japanese Patent Publication Sho 50-37009 and U.S. Pat. No. 3,841,924.